As is known, electromagnetic (EM) radiation can have a non-zero orbital angular momentum (OAM), and EM beams (e.g., radio frequency beams) with a non-zero OAM can be transmitted and received. FIG. 1 illustrates an example of a prior art OAM antenna system 100 comprising a feed 102 and a reflective antenna 112. Typically, the feed 102 illuminates the reflective face 104 of the antenna 112 with a non-OAM EM signal 114 (e.g., a radio frequency signal 114 with zero OAM). From a starting edge 116 to an ending edge 118, the surface of the face 104 twists and moves along an axis 110 such that there is a discontinuity 106 (e.g., a step) between the starting edge 116 and the ending edge 118. The foregoing shape can impart a non-zero OAM to the EM signal 114 as the EM signal 114 reflects off of the face 104 of the antenna 112. The beam 108 reflected from the antenna 112 can thus have non-zero OAM. That is, as shown in FIG. 1, the beam 108 can twist around the axis 110 as the beam propagates away from the face 104.
As illustrated in FIG. 2, multiple OAM beams 202, 204 (two are shown but there can be more) each in the same frequency band but having a different mode can be combined and transmitted as a composite transmission 200 from an EM transmitter (not shown). An EM receiver (not shown) can received the composite transmission 200 and separate the multiple OAM beams. The ability to combine multiple beams in the same frequency band provides for the possibility of very high data rate transmissions. Two OAM beams (e.g., each like one of beams 108, 202, 204) with the same mode but opposite polarization can also be combined at a transmitter and separated at a receiver.
Some embodiments of the present invention efficiently combine multiple OAM beams to transmit a composite OAM beam comprising a plurality of OAM beams each with a different mode and/or polarization.